Daytime convective development over land: A model intercomparison based on LBA observations

Abstract

SUMMARYThis paper investigates daytime convective development over land and its representation in single-column models (SCMs) and cloud-resolving models (CRMs). A model intercomparison case is developed based on observations of the diurnal cycle and convection during the rainy season in Amazonia. The focus is on the 6 h period between sunrise and early afternoon which was identified in previous studies as critical for the diurnal cycle over summertime continents in numerical weather prediction and climate models. This period is characterized by the formation and growth of a well-mixed convective boundary layer from the early morning temperature and moisture profiles as the surface sensible-and latent-heat fluxes increase after sunrise. It proceeds with the formation of shallow convective clouds as the convective boundary layer deepens, and leads to the eventual transition from shallow to deep precipitating convection around local noon. To provide a benchmark for other models, a custom-designed set of simulations, applying increasing in time computational domain and decreasing spatial resolution, was executed. The SCMs reproduced the previously identified problem with premature development of deep convection, less than two hours after sunrise. The benchmark simulations suggest a possible route to improve SCMs by considering a time-evolving cumulus entrainment rate as convection evolves from shallow to deep and the cloud width increases up to an order of magnitude. The CRMs featuring horizontal grid length around 500 m are capable of capturing the qualitative aspects of the benchmark simulations, but there are significant differences among the models. Two-dimensional CRMs tend to simulate too rapid a transition from shallow to deep convection and too high a cloud cover.

Variable cirrus shading during CSIP IOP 5. I: Effects on the initiation of convection

“…Grabowski et al, 2006). These modelling studies test the sensitivity of modelled CI to flux variations of similar magnitudes to those observed during CSIP IOP 5.…”

Time-dependent, non-monotonic response of warm convective cloud fields to changes in aerosol loading

“…The mean COG height of the water mass (Grabowski et al, 2006;Koren et al, 2009) (Fig. 3d), increases with the aerosol loading up to a relatively high concentration (500 cm −3 ).…”

Modelling convective processes during the suppressed phase of a Madden–Julian oscillation: Comparing single‐column models with cloud‐resolving models

“…When averaged over the three experiments, the SCMs are 3.8 kg m −2 drier after the first 48 hours when averaged over the three integrations, mostly as a result of larger surface precipitation in the SCMs. Results during this period are influenced by the 'spin-up' of the CRMs (the development of precipitating cumulus convection from a cloudless initial state) and by 'spin-down' in the SCMs (which often experience precipitation maxima soon after initialization (Grabowski et al, 2006)), although systematic errors in the representation of convective and precipitation processes could also play some role in these differences.…”

“…In this study, simulations are carried out using cloud-resolving models (CRMs), which explicitly resolve cloud-scale processes, and single-column model (SCM) versions of numerical weather prediction (NWP)/global climate models (GCMs), which parametrize all cloud processes on scales smaller than that of a GCM grid cell (∼ 100 km). Previous model intercomparison studies of deep convective cloud systems using CRMs and SCMs were based upon convectively active periods of field experiments that took place over oceans (Bechtold et al, 2000;Redelsperger et al, 2000) and land (Ghan et al, 2000;Xie et al, 2002;Xu et al, 2002;Guichard et al, 2004;Grabowski et al, 2006). In order to understand the deficiencies in the representations of convective processes in GCMs and, in particular, their ability to simulate tropical variability, the present case study includes both suppressed and active periods of tropical deep convection, as well as the transitions from suppressed to active periods during the suppressed phase of the Madden-Julian oscillation (MJO), which has a period of 40-50 days (Madden and Julian, 1972).…”